ISDN to VoIP: moving your ISDN phone system

Why ISDN phone systems are moving to VoIP

ISDN is a digital phone service delivered over copper. ISDN2 gives two channels, so two calls at once, and is common in small offices. ISDN30 is supplied in bundles of channels, up to 30 per bearer, and usually carries a range of direct-dial (DDI) numbers into an on-site PBX.

Openreach's advice to businesses is plain: "All services supported by ISDN need to be migrated to an IP service by January 2027 or your service will be ceased." That deadline sits alongside the move of all traditional lines to digital on 31 January 2027. The phone system itself can often carry on for a while: what has to change is how it connects to the outside world.

For the wider comparison of replacements, our sister site sets out the ISDN replacement options, and Forever Communications covers the ISDN switch-off timetable. This page focuses on the practical routes from ISDN to VoIP.

Three routes from ISDN to VoIP

Each route suits a different starting point. The age and support status of your PBX is usually what decides it.

Whichever route you take, size the new service on the calls you actually make, and plan the port of your DDI range early.

ISDN-to-SIP gateways: the ISDN VoIP converter

An ISDN-to-SIP gateway, sometimes called an ISDN VoIP converter or VoIP ISDN adapter, is a small box with ISDN ports on one side and a network port on the other. The PBX plugs into it exactly as it plugged into the ISDN line, using a basic-rate (ISDN2) or primary-rate (ISDN30) interface, and the gateway carries the calls over SIP trunks to the phone network.

To the PBX, nothing has changed, so staff keep their handsets, extension numbers, menus and hunt groups. That makes a gateway a sensible bridge when your PBX still meets your needs and you would rather replace it on your own timetable. Points to check before choosing one:

• Interfaces and channels. The gateway needs the right number of ISDN2 or ISDN30 ports and enough channels for your busiest moment.
• Number presentation. DDI digits and caller ID need mapping so inbound calls still reach the right extension and outbound calls show the right number.
• Fax, alarms and data devices that shared the ISDN service need their own plan.
• The PBX itself still needs support, security updates and eventual replacement.

SIP trunks straight into the PBX

Many PBXs can talk SIP, sometimes after a licence or card is added. In that case the ISDN lines can be replaced with SIP trunks directly, with no gateway in between. Our sister site explains how SIP trunk providers connect a phone system, and the difference between the protocol and the technology is covered in SIP vs VoIP.

Microsoft says Teams Direct Routing suits organisations that need integration with third-party PBXs or analogue devices. If some staff want to work in Microsoft Teams while the PBX stays, Dstny's Direct Routing service connects Microsoft Teams users to an existing SIP-capable PBX or SIP trunk with nothing installed on site; each user needs Teams Phone Standard. Our sister site explains Teams Direct Routing in more detail.

Moving to a new VoIP phone system

If the PBX is near the end of its life, or you want apps, home working and simpler administration, the ISDN switch is a natural moment to replace it. Forever Systems supplies:

• 3CX, licensed per system by simultaneous calls, hosted by 3CX, self-hosted in your own cloud account or on site
• Microsoft Teams Phone through Calling Plans, Operator Connect or Direct Routing, including Forever Teams, our Direct Routing service that keeps your existing numbers
• Webex Calling, Cisco's cloud calling service, with the PSTN connection chosen per location
• Forever UC, Forever Group's own app-first platform with optional desk phones, in Standard, Plus and Contact tiers

BT Cloud Voice is also available, supplied by our sister company Forever Communications through cloudvoice.cloud. Your extensions, groups and menus are rebuilt on the new platform, and VoIP desk, cordless and conference handsets can be bought outright or, in some cases, included in your monthly rental, with the options confirmed at quote. PBX vs VoIP explains how the old and new systems compare.

DDI ranges and how channels map to calls

DDI ranges. An ISDN30 service usually carries a block of direct-dial numbers. The range can normally be ported with its main number to the SIP trunk provider or new platform, so callers keep dialling the same direct numbers. Include the whole range and the main number on the port request; port your landline number to VoIP explains the letter of authority and matching details.

Channels to simultaneous calls. One ISDN channel carries one call, so an ISDN30 bundle of a given size handles that many calls at once. On VoIP the equivalent is the number of SIP channels or, on 3CX, the simultaneous calls (SC) in the licence. Size it on your peak simultaneous external calls from your call logs, plus headroom, rather than copying the old channel count, which often included spare capacity. Our sister site's guide to how many SIP channels you need walks through the sums.

Bandwidth. Each call also needs bandwidth. Cisco's figures, including network overhead, are about 87.2 kbps in each direction per G.711 call and about 31.2 kbps per G.729 call, so check the connection against your peak calls with our VoIP requirements checklist.

Planning your ISDN to VoIP move

1. Check the PBX. Note its age, who maintains it, whether it is still supported and whether it has SIP or ISDN interfaces.
2. Measure peak calls. Use call logs to find your busiest moment, including queues and conference calls.
3. List everything on the ISDN service. DDI ranges, fax, alarms, card terminals and any data devices.
4. Check connectivity. Confirm each site's connection can carry your peak calls, with a backup path if calls are business-critical.
5. Choose the route. Gateway, SIP trunks or a new VoIP phone system, based on the steps above.
6. Port the numbers and test. Agree a porting date, test every DDI and outbound caller ID, then cease the ISDN service.

Contact us with your current set-up and peak call volumes, and we will compare the routes for your sites.